For amplifying an audio signal, an analog amplification system and a digital amplification system are used. The analog amplification system converts a pulse code modulation (PCM) audio signal into an analog signal and performs current amplification and voltage amplification having an intermediate voltage stage, so amplification efficiency decreases. On the other hand, the digital amplification system performs pulse width modulation on the PCM audio signal and also performs switching amplification by repeating ON/OFF, and the PCM audio signal is output through an analog filter. Therefore, the digital amplification system has amplification efficiency greater than that of the analog amplification system. In addition, the amplification circuit of the digital amplification system can be embodied in a small size and has low power consumption. There are many kinds of the digital amplification systems. In a digital differential drive system used in the digital amplification system, there are also many kinds of systems including an AD drive system and BD drive system.
FIG. 1 is a view showing a pulse width modulation circuit in a conventional AD drive system;
Referring to FIG. 1, a PCM audio signal is input to a (+) port of a comparator 100, and a carrier signal is input to a (−) port of the comparator 100. The carrier signal, for example, has a triangle waveform. The comparator 100 outputs a value of +1 or −1 according to a result of comparing the two signals input to the (+) port and the (−) port. For example, if a value of the PCM audio signal input to the (+) port is larger than a value of the carrier signal input to the (−) port, a value of +1 is output, otherwise, −1 is output. Accordingly, output waveform of a PWM+ signal, which is a positive PWM signal, is a pulse width modulation signal having a different pulse width according to a value of the input PCM audio signal. A PWM− signal, which is a negative PWM signal, is the inverted PWM+ signal by an inverter 102.
FIG. 2 is a view showing a pulse width modulation circuit in a conventional BD drive system;
In the BD drive system, two comparators 110 and 112 are used, a PCM signal input to a (+) port of the comparator 110 is inverted by an inverter 114 and input to a (+) port of the other comparator 112. The same Triangle waveforms are used for carrier signals in both comparators 110 and 112. Therefore, a PWM+ signal, which is a positive PCM signal, and a PWM− signal, which is a negative signal, are output.
FIG. 3 is a view showing a pulse width modulation output signal according to the conventional BD drive system;
The PCM audio signal 200 is input to the (+) port of the comparator 110, and the carrier signal 210 is input to the (−) port of the comparator 110. The comparator 110 outputs a value of +1 or −1 according to a result of comparing the two signals input to the (+) port and the (−) port, so that the PWM+ signal 220 which is a positive PWM signal is output. The inverted PCM audio signal 200 is input to the (+) port of the other comparator 112, so the PWM− signal 230 which is a negative PWM signal is output. Referring to FIG. 3, modulated pulse widths of the PWM+ and the PWM− signals 220 and 230 vary according to a value of the PCM audio signal.
When the PWM+ and the PWM− signals 220 and 230 are connected to both terminals of load, a differential output 240 subtracting the PWM− signal 230 from the PWM+ signal 220 is applied to the load. A waveform of the differential output 240 is the same as the output waveform denoted by reference numeral 240 shown in FIG. 3.
According to the conventional method of pulse width modulation, when the input PCM audio signal is 0, bias currents of the load connected to the differential output can be controlled. However, residual noises due to switching exist in each of the PWM+ and the PWM− signals. Further, some currents flow through a common ground, and unnecessary heat is generated. Thus, amplification efficiency decreases, and pop noises, which may damage a speaker and offend a user's ear, are generated when the amplifier operates.